1. When a phrase means the opposite of what it literally says.
Opp. literalNB However similar, not to be confused with sarcasm

2. When an event happens in a certain situation which is contrary to what one would expect.

It would be ironic if the former Worlds Fattest Man died of starvation.

When Ricky Martin, latin american sex-symbol and ladykiller, became famous, there were rumours flying about the media that he was gay.
However, when the russian group T.a.T.u became famous, rumours arose in the media that the two girls that were alledgedly lesbians were in fact heterosexual.